In typical gas discharge lasers such as the excimer laser described in U.S. Pat. Nos. 5,377,215 and 5,748,656 two electrodes are disposed in a spaced apart relationship in a chamber and a gas is circulated between the electrodes. FIG. 1 is a cross section drawing of a prior art KrF excimer laser as shown in U.S. Pat. No. 5,377,215. When a high voltage pulse is applied between the electrodes, 18 and 20 in FIG. 1, an electrical discharge is created which produces a laser pulse. In prior art commercial excimer lasers pulses may be produced at rates in the range of 100 Hz to 2000 Hz to produce a pulsed laser beam. The optical energy in a typical pulse of a commercial KrF excimer laser used in integrated circuit lithography is about 10 mJ and the electrical energy utilized to produce it is somewhat more than 2J most of which heats laser gas circulating in the laser chamber. So if the laser is operating continuously at pulse rate of 1000 Hz, the discharges are dumping about 2,000 watts of energy into laser chamber 10. Only about 0.5 percent of the input energy comes out as the laser pulse. The laser gas, which in this case is a mixture of about 0.1% fluorine, 1% krypton and the rest neon, is circulated between the electrodes at a speed of about 2000 cm per second by 3.25 inch tangential blower fan 50 operating at about 3300 rpm. This fast circulation of the laser gas by the fan adds heat to the chamber at the rate of about 500 watts. Performance of the laser is known to be affected by gas temperature and it is known to experiment with lasers to determine a temperature which results in optimum performance. Preferred operating temperatures of a typical prior art KrF excimer laser is in the range of about 50.degree. C.
In order to operate at a desired temperature such as 50.degree. C., some prior art lasers utilize heating elements which may be mounted on or in the chamber wall to help initially increase the temperature to the desired range and to increase the chamber temperature if the temperature falls below a desired range. Cooling in prior art lasers is typically provided by a finned water cooled heat exchanger, such as 66 as shown in FIG. 1, disposed inside the chamber. In some cases additional cooling is provided by water cooled cold plate mounted on the outside of the chamber walls. Cooling systems must on average remove heat from the chamber at the same rate it is added to the chamber in order to maintain a constant average temperature.
A preferred operating mode for an excimer laser used as a light source in stepper and scanner equipment for integrated circuit production is called a "burst mode". In this mode the laser is operated in "bursts" such as about 300 pulses at a pulse rate of 1000 Hz, each 300 pulse burst illuminates a single exposure site of about 2 or 4 square centimeters on a wafer which may be about 8 inches in diameter. There are typically many (such as about 85) exposure sites on a single wafer each site corresponding to one or more integrated circuits. Each burst is separated by an idle period, for example, of 0.3 seconds. After, for example, 85 "bursts" covering 85 exposure sites, the laser may be idle for a longer period of, for example, about 9 seconds while a new wafer is moved into position by the stepper or scanner.
FIG. 2 is a modified version of FIG. 3 from U.S. Pat. No. 5,748,656 (with most of the designation numbers omitted) and it shows a prior art system for controlling temperature. The system comprises a temperature detector 330 extending into the chamber to measure gas temperature. The signal from detector is utilized by a microprocessor based controller in a feedback loop to control water flow to cooling finned heat exchanger 66 and rod type heating elements 67 in order to maintain the gas temperature within a desired operating range. That patent teaches that the processors can be programmed to cause a heating element to add heat equivalent to the heat which would have been added by discharge pulses during short idle times so that approximately constant gas temperatures can be maintained. With the heater controlled by the microprocessor, heat could be added whenever the laser is not operating continuously (such as during the above 0.3 second down time) or, more practically, in between separate series of bursts (such as the above 9 second idle period).
Adding heat energy with a heating element equivalent to about 1000 watts during the 9 second idle period provides substantially constant heat input and heat extraction without varying the cooling water flow. Alternatively, some prior art systems vary the cooling water flow based on a temperature feedback arrangement in order to maintain gas temperature within an operating range. A problem with these approaches is that whereas heat is added to the laser gas instantaneously by the electrodes, several seconds or minutes may be required for heat to be transferred from a heating element to the laser gas. Several seconds or minutes may also be required for changes in water flow to be effective. Therefore, temperature fluctuations (even oscillations) of a few degrees C. about a desired temperature generally result from the normal burst mode operation in prior art gas discharge lasers. These temperature fluctuations can adversely affect performance.
What is needed are improvements which will permit better temperature control of electric discharge gas lasers.